You may already be aware of the discussion of the Hallar family in W.Z. Hickman's 1920 History of Jackson County, Missouri. It's searchable online under Google searches. Page 353 on the Hallars. Says that Oliver J. Hallar served in Price's army and John Colvin was for some years a pony express rider. They were brothers of William M. Hallar (Bill Haller--see Bruce Nichols Vol. 1) who was a lieutenant of Quantrill and of James Albert Hallar who was killed on the Sniabar, according to this history.
The Missouri Soldiers Data Base says Oliver J. Hallar joined the 2nd Mo. Cav. CSA in 1861 at Independence, but there is no record of him among the regimental records online.
Tom Jones